In recent years, OCT that forms images of the surface morphology and internal morphology of an object by using a light beam from a laser light source or the like has attracted attention. Unlike an X-ray CT apparatus, optical coherence tomography is noninvasive to human bodies, and is therefore expected to be utilized in the medical field and biological field. For example, in the ophthalmology, devices that form images of a fundus and cornea or the like are in a practical stage.
Patent Document 1 discloses a device to which OCT is applied. This device has such a configuration that: a measuring arm scans an object by a rotary deflection mirror (a Galvano mirror); a reference arm is provided with a reference mirror; and an interferometer is mounted at the outlet to analyze, by a spectrometer, the intensity of an interference light of light fluxes from the measurement arm and the reference arm. Moreover, the reference arm is configured to gradually change the light flux phase of the reference light by discontinuous values.
The device of Patent Document 1 uses a technique of so-called “Fourier Domain OCT.” That is to say, the device irradiates a low-coherence light beam to an object, superposes the reflected light and the reference light to generate an interference light, and acquires the spectral intensity distribution of the interference light to execute Fourier transform, thereby imaging the morphology in the depth direction (the z-direction) of the object. The technique of this type is also called Spectral Domain.
Furthermore, the device described in Patent Document 1 is provided with a Galvano mirror that scans with a light beam (a signal light), and is thereby configured to form an image of a desired measurement target region of the object. Because this device is configured to scan with the light beam only in one direction (the x-direction) orthogonal to the z-direction, an image formed by this device is a two-dimensional tomographic image in the depth direction (the z-direction) along the scanning direction (the x-direction) of the light beam.
Patent Document 2 discloses a technique of scanning with a signal light in the horizontal direction (x-direction) and the vertical direction (y-direction) to form a plurality of two-dimensional tomographic images in the horizontal direction, and acquiring and imaging three-dimensional tomographic information of a measured range based on the tomographic images. As the three-dimensional imaging, for example, a method of arranging and displaying a plurality of tomographic images in the vertical direction (referred to as stack data or the like), and a method of executing a rendering process on a plurality of tomographic images to form a three-dimensional image are considered.
Patent Documents 3 and 4 disclose other types of OCT devices. Patent Document 3 describes an OCT device that images the morphology of an object by sweeping the wavelength of light that is irradiated to an object, acquiring the spectral intensity distribution based on an interference light obtained by superposing the reflected lights of the light of the respective wavelengths on the reference light, and executing Fourier transform. Such an OCT device is called a Swept Source type or the like. The Swept Source type is a kind of the Fourier Domain type.
Further, Patent Document 4 describes an OCT device that irradiates a light having a predetermined beam diameter to an object and analyzes the components of an interference light obtained by superposing the reflected light and the reference light, thereby forming an image of the object in a cross-section orthogonal to the travelling direction of the light. Such an OCT device is called a full-field type, en-face type or the like.
Patent Document 5 discloses an example of applying OCT to the ophthalmologic field. In addition, before OCT was applied, a retinal camera, a slit lamp, etc. were used as devices for observing an eye (e.g., see Patent Documents 6 and 7). The retinal camera is a device that photographs the fundus oculi by projecting illumination light onto the eye and receiving the reflected light from the fundus oculi. The slit lamp is a device that obtains an image of the cross-section of the cornea by cutting off the light section of the cornea using slit light.
The device with OCT is superior relative to the retinal camera, etc. in that high-definition images can be obtained, further in that tomographic images and three-dimensional images can be obtained, etc.
Optical image measurement apparatuses that apply OCT include those provided with an optical attenuator (sometimes simply referred to as “attenuator”) for adjusting light amount (intensity). For example, in Patent Document 8, an attenuator is described that attenuates respective wavelength bands at different attenuation factors to make the light amount uniform. This attenuator adjusts the attenuation factors using a rotatable neutral density filter.
In addition, it is common for attenuators to shield light with a light-blocking material (e.g., see Patent Document 9). Moreover, in Patent Document 9, a configuration is disclosed that uses two attenuators disposed facing across a parallel beam. The purpose of this configuration is to eliminate any limitations on the direction of travel of light beams and to uniformly shield light beams.